This invention relates to an instrument for measuring properties of a surface coating. It further relates to apparatus, including the instrument, for the determination of average coating thickness. Particularly, but not exclusively, the surface can be a rough moving surface.
There are many instances in industry where coatings are applied to surfaces. Some of these coatings are of a protective nature during the manufacturing process, whilst others are to protect or enhance the surfaces in their post-production applications. Other related fields may include printing and film production methods involving, for example, the application of dampening solution to printing drums.
Taking, by way of specific example, the production of aluminium can stock, a coating of oil is required on each side of the aluminium for lubrication. If too little oil is applied to either side there will be excessive friction between the drums and the processed aluminium, resulting in imperfections in the aluminium produced. If an excessive amount of oil is deposited on the aluminium, lubricant is wasted and must be removed prior to further processing.
It is desirable to be able to measure the thickness of the oil coating that is being applied to the aluminium during production to ensure that the optimum amount of lubrication is being applied.
It is known to use a weighing technique to determine the average thickness of the oil coating. The surface is weighed before and after being coated, thus determining the weight of the oil coating. It is then possible to determine the average thickness of the coating using the weight of the oil in conjunction with the density of the oil and the surface area of the material that is being coated.
The current weighing technique is flawed as it can be significantly influenced by the presence of a few oil droplets. If such oil globules are, indeed, present, they may constitute a major part of the oil mass on the surface being examined, particularly when considering thin film coatings. Subsequently, oil droplets make the average thickness unrepresentative of the true surface coating. Furthermore, as the technique requires the sample to be weighed before and after cleaning, it is not practical for real-time monitoring of the coating thickness during application of the coating. The measurement is done offline with, typically, a 1 m2 sample taken every several kilometers of strip.
Close examination of the surface of the aluminium can stock has shown that the surface is rough. It has been determined that the oil does not cover the surface evenly; rather, the coating fills the troughs present in the surface of the aluminium stock first, whilst the peaks may remain bare.
It is an object of the present invention to determine at least the thickness of a liquid or solid coating on a surface. This object may be extended to determine the weight of a coating.
The invention discloses an apparatus for measuring the average thickness of a non-uniform coating on a rough surface, comprising:
a source of light of substantially a single wavelength, which is to be directed at a coated surface being examined;
two or more photodetectors for measuring the intensities of different polarisation states of said light that is reflected from the surface; and
processing means for calculating the average coating thickness from said polarisation state intensities.
The invention further discloses an apparatus for measuring average thickness of a non-uniform coating on a rough surface comprising:
a source of light of substantially a single wavelength, which is to be directed at a surface being examined;
a polarising prism beamsplitter arranged to receive a reflected beam from said surface and produce an orthogonal pair of reflection components at xc2x145xc2x0 to the plane of incidence of said light source;
a pair of photodetectors each receiving a respective one of said reflection components and producing respective intensity signals thereof; and
processing means for calculating the average coating thickness from said polarisation state intensities.
The invention yet further discloses a surface coating measurement instrument comprising:
a housing, within which is arranged:
(a) a light source of substantially a single wavelength, which is to be directed at a coated surface being examined;
(b) a transmission aperture from which said light emanates;
(c) a reflection reception aperture through which reflected light passes;
(d) a polarising prism beamsplitter receiving said reflected light and arranged to produce an orthogonal pair of reflection components at xc2x145xc2x0 to the plane of incidence of said light; and
(e) a pair of photodetectors for measuring the intensities of the different polarisation states from the beamsplitter.
The invention yet further discloses a method for measuring the average thickness of a non-uniform coating on a rough surface, the method comprising the steps of:
directing the beam of light of a substantially single wavelength at a coated surface being examined, said beam being at non-normal incidence to the surface;
measuring the intensities of the different polarisation states of the beam reflected from the said surface coating, using a plurality of photodetectors; and
determining the average thickness of the non-uniform coating from the polarisation state intensities.
The invention yet further provides a method for measuring the average thickness of a non-uniform coating on a rough surface, the method comprising the steps of:
directing the source of light of a substantially single wavelength at a coated surface being examined, said beam being at non-normal incidence to the surface;
measuring the intensities of polarisation states of said light for xc2x145xc2x0 components to the plane of incidence of said light source; and
determining the average thickness of the non-uniform coating from the polarisation state intensities.
The invention can be practised on moving or static rough surfaces alike. For a static surface, if the instrument or sample is scanned it is possible to form a surface mapping of film thickness.